i) Field of the Invention
The present invention relates to a refiner force sensor for refiners used in the pulp and paper industry, to a refining apparatus, and to a method of measuring forces acting on a refiner bar in a refiner.
ii) Description of Prior Art
Refiners are used to produce pulp from wood chips or to modify the mechanical properties of wood fibres by repeatedly applying forces to the material processed by means of bars mounted on two opposing surfaces that move relative to one another.
Refiners are commonly used in the pulp and paper industry to repeatedly subject wood fibres or wood chips to stresses and strains. In the case where wood chips are processed, the purpose is usually to separate wood fibres from one another to produce pulp that can later be used to manufacture paper or composite wood products such as hardboard. This process is generally conducted at high temperature and pressure in a steam environment, because a large amount of steam is produced in the refiner from the heat dissipated while processing the material. Coarse pulps produced in such a way can also be further processed in a similar way to improve some of the properties of fibres. Examples of this are the commonly used practice of subjecting pulp to a second stage of refining, or to screening followed by reject refining. Low-consistency or flow-through refiners are also used to process pulp slurries at consistencies up to approximately 5%. In this case, the aim is generally to stress and strain wood fibres in order to improve some of their properties.
A vast array of operating conditions are used in industrial refining systems, but a number of design features are common to all refiners. Refiner discs are fitted with plates having alternating patterns of bars and grooves. The bars of opposing plates are separated by a small gap that can be adjusted, and at least one of the discs rotates. Pulp travels through a refiner in the form of fibre agglomerates that are repeatedly compressed and sheared between the bars of opposing plates as these travel past each other. Hence, all refiners expend energy on fibres through a repeated application of compression and shear forces acting on fibre agglomerates.
To quantify the effects that these forces have on the individual pulp fibres, some measure of the degree of refining must be taken. Traditionally, this measure has simply been the specific energy, which is the total energy put into the pulp per oven dry mass of fibre. However, it is widely known that this parameter is not sufficient to fully characterize the refining action, since vastly different pulp properties can be obtained at the same level of specific energy under different refining conditions. Several methods have been proposed to use an additional parameter to characterize the action of refiners. The additional parameter usually aims to quantify the severity of bar impacts. This is achieved in different ways with each method, but the severity of bar impacts is generally expressed as a specific energy per impact. However, energy-based characterizations have shortcomings when it comes to identifying the mechanisms by which refining occurs. Energy can be expended on pulp fibres in numerous ways and the method of energy application—the forces—can have a substantial influence on the final pulp properties. Giertz, H. W. (“A new way to look at the beating process”, Norske Skogindustri 18(7):239-248, 1964) suggested that different refining effects could be explained by the relative magnitude of the forces applied. Similarly, Page, D. H. (“The beating of chemical pulps—The action and the effects”, In Fundamentals of Papermaking: Transactions of the Fundamental Research Symposium held at Cambridge, F. Bolam editor, Fundamental Research Committee, British Paper and Board Makers' Association, Volume 1, pp. 1-38, 1989), has suggested that a complete understanding of the refining process would require knowledge of the average stress-strain history of individual fibres.
Early work on forces focused on measuring the pressure on refiner bar surfaces. Two of these studies were in low-consistency applications (Goncharov, V. N., “Force factors in a disk refiner and their effect on the beating process”, English translation, Bum. Promst. 12(5):12-14, 1971; and Nordman, L., Levlin, J. -E., Makkonen, T., and Jokisalo, H., “Conditions in an LC-refiner as observed by physical measurements”, Paperi ja Puu 63(4): 169-180, 1981), while one was at high consistency (Atack, D., “Towards a theory of refiner mechanical pulping”, Appita Journal 34(3):223-227, 1980). The harsh conditions that exist within the refining zone of commercial refiners have proven too severe for standard pressure sensors. These generally fail within a few minutes of operation in these conditions.
Despite the shortcomings of standard pressure sensors, a method has been proposed by Karlström (International Patent Publication No. WO 97/38792) to use them, in conjunction with temperature sensors, to regulate the operation of high-consistency chip refiners. In the control scheme proposed, the mass flow rate of chips and the dilution water flow rate to the refiner, as well as the pressure applied to regulate the gap between refining discs, are adjusted in response to measured values of pressure and temperature in the refining zone. The aim of the method is to control the temperature and the pressure profile across the refining zone in order to maintain desired values of these parameters. WO 97/38792 also claims a method to control specific pulp properties by raising or lowering the temperature in the refining zone. In International Patent Publication No. WO 98/48936, Karlstrom proposes an arrangement of such temperature and pressure sensors for installation in a refiner. WO 97/38792 and WO 98/48936 relate only to the chip refining process.
The pressure measured in the way prescribed by the above method is not due directly to mechanical forces imposed on pulp in the refining zone. It is rather due to the presence of steam produced as a result of the large amount of mechanical energy expended in the refiner that is dissipated as heat. While the steam pressure depends on the amount of energy dissipated locally in the refining zone, it is also strongly dependent on the ease with which steam can escape the refiner along the radial direction.
U.S. Pat. No. 5,747,707 of Johansson and Kjellqvist proposed the use of one or more sensor bars in a refiner. The sensor bars are equipped with strain gauges to measure the load at a number of points along their length. By mounting several strain gauges at each point, the authors suggest that the stresses on a bar can be divided into load components acting in different directions. The apparatus can also include temperature gauges that can be used to compensate the measured stresses for thermal expansion of the bar. In another embodiment, the apparatus includes means for controlling refining in response to the load determined by the sensors.
A sensor bar with a design similar to the one described in the above U.S. patent was used by Gradin et al. (Gradin, P. A., Johansson, O., Berg, J. -E., and Nystrom, S., “Measurement of the power distribution in a single-disc refiner”, J. Pulp Paper Sci., 25(11):384-387, 1999) to measure the distribution of the expended power in the refining zone of a single-disc refiner. The authors found that the power expended per unit area was approximately constant over the radius of the refining zone. This confirmed an earlier finding of Atack, D., and May, W. D. (“Mechanical reduction of chips by double-disc refining”, Pulp Paper Mag. Can. 64 (Conv. issue): T75-T83, T115, 1963). In order to improve the sensitivity of the sensor bar, the latter was manufactured out of aluminum. This choice of material is inadequate for long-term operation in an industrial refiner, since the sensor bar would wear much faster than the other refiner bars made of hardened material.